The field of the invention is thermoplastic-photoconductive recording medium. Photoconductive and thermoplastic material are generally applied over transparent conductive electrodes consisting of, for example, indium-tin oxide, indium oxide, nickel chromium, gold, platinum or aluminum, for use as an erasable holographic recording medium. Charge leakage can occur at a photoconductor-conductive electrode interface which reduces the charge contrast, and thus, causes poor performance of the thermoplastic-photoconductive device. The present invention teaches the use of a suitable blocking layer such as polyvinyl alcohol or polyvinyl butyral between the conductive layer and the photoconductive layer.
In the prior art it is known to use a blocking layer between the conductive and photoconductive layer. The U.S. Pat. No. 3,819,369 teaches for the blocking layer the use of phenoxy or epoxy resin with a thickness of between 200 and 400 Angstroms. The U.S. Pat. No. 3,953,207 indicates the layer may comprise any suitable transparent organic polymer or non-polymeric material capable of supporting the injection of photo excited holes from the photoconductive layer. That patent then lists a number of suitable polymers.
In the present invention it is desirable to use a material for the blocking layer which is not soluble in the photoconductor solvent. Such is true of polyvinyl alcohol and polyvinyl butyral. This is not true of the prior art epoxy or phenoxy resins which are soluble in the same solvent used for the photoconductor so that as the layer of photoconductor is applied it tends to dissolve away the blocking layer.